There is a growing need for receiving information from sources that are dispersed about a wide area. For example, for efficient farming and irrigation, knowledge of soil moisture content at various locations across a field or region is important; for efficient shipping and navigation, knowledge of sea conditions at various locations across an ocean is important. Similarly, there is a growing need for the control of devices that are dispersed about a wide area, such as switches that open or close irrigation flues. There is also a growing need for receiving information about the location of resources that may travel about a wide area. For example, for efficient wildlife or herd management, knowledge of the location of members of the herd is important; for property recovery, the knowledge of the location of a stolen vehicle is important. Such information and control messages are characterized as being of relatively short duration, and/or not necessarily time-critical. The information content of a particular message may also be relatively insignificant, but the aggregate information content from a plurality of remote sensors, such as barometric sensors, may have significance. However, because these control devices and information sources are dispersed over a large area, the communication of these relatively short and somewhat non-critical messages is often cost prohibitive using conventional communication systems. The use of relatively complex devices, such as cellular telephones or pagers, to communicate the messages also make the collection or distribution of these messages cost prohibitive.
Satellites offer the possibility of providing communications to and from remote terminal units over a wide service area, without the infrastructure typically required for ground-based communications systems. Because of the desire to keep the complexity of each satellite to a minimum, satellites also provide the opportunity to provide communication services customized to an anticipated type of information transfer. That is, a satellite communication system optimized for a particular type of message transfer, such as a high-volume of low-priority short-messages, will be significantly less complex, and therefore more inherently reliable and less costly than conventional systems designed for high-priority continuous information transfer.
To communicate via a satellite, the transmitted signal from a ground station must be received at the satellite at a sufficient signal to noise ratio (SNR), and the retransmitted signal from the satellite must be also be received at the intended ground station at a sufficient SNR. The SNR can be increased by increasing the power density of the signal being received, or by reducing the power density of the noise being received. To optimize the received power density, directional antennas are used to narrow the transmission beamwidth, thereby increasing the portion of the transmitted power being received by the receiver by minimizing the dispersion of the transmitted power. Because the transmitted power density within a narrow-beamwidth antenna's beamwidth is increased, as compared to the transmitted power density from an omnidirectional antenna, a narrow-beamwidth antenna is termed a high-gain antenna; a wide-beamwidth antenna is termed a low-gain antenna. Directional, high-gain antennas are used to narrow the receiver beamwidth, to decrease the portion of noise energy being received. Directional high-gain antennas must be aimed so that the intended receiver antenna or transmitter antenna is contained within the narrowed beamwidth. The narrower the beamwidth, the more precise the aiming must be. The area encompassed by an antenna's beamwidth is termed the antenna's field of view.
To minimize the number of satellites needed to provide communications over a wide geographic area, each satellite should have an antenna with a field of view that covers a maximum amount of the satellite's service area. That is, each satellite should have a relatively low-gain wide-beamwidth antenna. To provide a high signal to noise ratio for communications to and from the satellite, the ground station uses a high-gain narrow-beamwidth antenna, to compensate for the satellite's low-gain antenna. Satellites that broadcast television signals, for example, utilize a relatively wide-beamwidth antenna covering their entire service area, and each television receiver requires a high-gain narrow-beamwidth antenna that is aimed at the transmitting satellite.
The requirement to aim a directional high-gain antenna at a satellite is infeasible or impractical for mobile ground terminals, or for satellites that are moving relative to the ground terminal. This requirement also increases the cost of the ground terminals, making their use for relatively infrequent and low-priority messages cost prohibitive. The aforementioned satellite television broadcast system uses geo-stationary satellites, and is intended for fixed reception sites. Geo-stationary satellites are significantly more expensive to launch and maintain than lower altitude satellites, and, being farther from the earth, require more transmitted power or higher-gain antennas. A typical solution for mobile ground terminals and moving satellites is to use a narrow-beam high-gain antenna at the satellite, and allow wide-beam antennas at the ground terminals. The use of narrow-beam antennas, however, requires a significant increase in the number of satellites needed to provide communications over a large geographic area, because each antenna's field of view is significantly smaller than the satellite's service area, and overlapping satellite service areas are required to provide sufficient fields of view that cover the geographic area. As with ground communications systems, however, providing a significant number of satellites to a sparsely populated geographic area may not be economically feasible, and the cost of providing such a service to an economically disadvantaged region may preclude its use. Furthermore, in populated areas, the profusion of mobile telephony and high speed data transfer communications imposes significantly complex design requirements on all transmitters, such that the cost of using existing systems for the transmission of relatively short bursts of information or control messages is not justified.